Thermal ink-jet printheads typically incorporate a plurality of electrical resistance elements on a common substrate for the purpose of heating ink in adjacent reservoirs in order to vaporize a component of the ink composition. The vaporized component of the ink composition imparts mechanical energy to a quantity of ink thereby propelling the ink through one or more orifices in an orifice plate of the ink-jet printhead toward a print medium in a predefined sequence to form alphanumeric characters and graphics thereon.
In order to provide better print quality, many electrical resistance elements and orifices are provided on a single ink-jet printhead. As the number of electrical resistance elements and orifices on the printhead increase, so does the print quality. However, increasing the number of electrical resistance elements and orifices on a single ink-jet printhead also increases the manufacturing difficulties associated with alignment tolerances of photomasks which must be maintained during fabrication in order to etch the substrate in the desired manner.
On a micro-scale, the ink-jet printhead must be precisely manufactured so that the components of the printhead cooperate to achieve the desired function and give the desired print quality. Hence, alignment of the ink feed slots, electrical resistance elements and orifices is critical to the proper operation of the ink-jet print head. The ink feed slots provide ink from a reservoir to the electrical resistance elements during the printing process. Since the printheads are precise micro-structural devices, even minor deviations or manufacturing difficulties during production of the ink-jet printhead components may result in a loss of useable substrate material and thus a low product yield.
One of the manufacturing technique used for forming ink feed slots in a silicon substrate of a thermal ink-jet printhead is an anisotropic etching technique. In this process, a silicon wafer having parallel (100) crystallographic planes is anisotropically etched to produce an elongated slot having a length ranging from about 3 to about 5 millimeters, a width ranging from about 0.5 to 2 millimeters and side walls which are at an angle of about 54.7.degree. from the planar surface of the silicon wafer. Prior to completion of the printhead, electrical resistance elements and electrodes are attached to one surface of the silicon substrate adjacent the ink feed slots. Manufacturing difficulties are often encountered when attempting to precisely position the feed slots and electrical resistance elements relative to one another.
U.S. Pat. No. 5,387,314 to Baughman et al. discloses a method for making ink fill slots in a silicon substrate. The disclosed procedure includes a partial anisotropic etch from one surface of the silicon substrate whereby the fill slots are etched only part way through the substrate. In a subsequent step, an isotropic etchant is used to complete the fill slots from the opposing surface of the substrate. According to Baughman et al., isotropically etching the silicon from the opposing surface of the substrate reduces the distance from the ink fill slot to the entrance of the ink feed channel. While the method Baughman et al. may reduce the effect of alignment problems in the manufacture of ink fill slots relative to the electrical resistance elements, it requires that the fill slots be extended to the firing chamber on the opposing surface of the substrate by use of a subsequent masking and isotropic etching step. Thus, the procedure requires a combination of etching procedures with multiple alignment of photo-masks which may make the manufacturing of the ink-jet printheads more difficult, costly and subject to alignment errors.
U.S. Pat. No. 5,308,442 to Taub et al. relates to another method for making printhead structures for introducing ink into the firing chambers of the printhead. In this process, a fragile membrane layer having a thickness of about 1 to 2 microns of dielectric material covers an etched ink fill slot until the resistors are formed and then the membrane is removed. As with many other manufacturing processes, the substrate having a membrane covering the ink fill slot must be handled with extreme care in order to avoid puncturing the membrane before the resistors are formed on the surface of the substrate. At high production rates, the yield of product using this technique may be unacceptably low.
U.S. Pat. No. 4,789,425 to Drake et al. relates to yet another method for fabricating thermal ink-jet printheads. The method disclosed by Drake et al. requires the use of etched alignment holes for use in patterning the silicon substrate for the fill slot etching process and for locating the position of the electrical resistance elements on the circuit side of the silicon substrate.
In the disclosed procedure, Drake et al. first patterns then partially or completely anisotropically etches the alignment holes and partially etches the reservoir/fill slots in the substrate. After partially etching the reservoir/fill slots, the resistance circuits are formed on the wafer. In another embodiment, Drake et al. completely etches the alignment holes through the substrate, the resistance circuits are formed then passivated, and the reservoir/fill slots are then patterned and etched in the substrate. Accordingly, Drake et al. require several critical aligning and patterning steps for locating the alignment holes, reservoir/fill slots and electrical resistance elements.
Since alignment steps are often performed manually, the use of multiple alignment steps adds to the labor costs, increases failure rate and slows down the production rate of the etched substrate parts. As the print speed and print quality of the ink-jet printers is increased, the fabrication tolerances become even more critical making the use of multiple masking and etching steps even less reliable for locating the position of the reservoir/fill slots and electrical resistance elements.
An object of the present invention is to provide an improved method for making ink feed slots for ink-jet printheads.
Another object of the invention is to improve the fabrication technique for forming ink feed slots for use in ink-jet printheads whereby the yield of acceptable product is increased.
A still further object of the invention is to provide an improved method for increasing the accuracy of locating electrical resistance elements relative to the ink feed slots of an thermal ink-jet printhead.
Another object of the invention is to reduce alignment difficulties and process steps thereby decreasing the time and increasing the yield of useable substrates for thermal ink-jet printheads.